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But how do you arrive at a CR under 12:1? Simple math (perhaps too simple?) suggests that 200 psig (214 psia) divided by 14 psi (our atmospheric pressure) equals 15.28:1. I am a gas compression guy, and while air isn't an Ideal Gas, the trusted and true P1*V1=P2*V2 suggests I am not far off.
Keith, it is called adiabatic compression. Wikipedia has a good website on it. As the gas is being compressed it heats up and increases the pressure beyond what the ratio of volumes would give. The P*V=const relationship is only valid for slow compression, where the added heat can be dissipated into the cylinder walls and the gas temperature stays constant.
For a diatomic gas (nitrogen, oxygen) for fast compression the formula P*V^(7/5) applies. For a CR=10, this would give a whopping 25x increase in pressure, but due to factors mentioned before (valve overlap, leaks, incomplete cylinder filling) the actual increase is much smaller.
Thomas
 
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